Dane -- who shot to fame amid six seasons on the hit ABC drama "Grey's Anatomy," where he played Dr. Mark Sloan, affectionately known as "McSteamy" -- said his symptoms began over one year ago, when he began to experience weakness in his right hand.

The "Euphoria" and "Grey's Anatomy" star, who revealed his diagnosis last Tuesday, sat down with "Good Morning America" to talk about his incurable condition.